Groundwater is an important water source. In consideration of the purity of the water, there are thresholds of acceptable tolerance for various metal ions. Amounts of metal ions dissolved in water that are above desirable or acceptable limits can be considered contamination. Heavy metal ions are particularly undesirable contaminants in many cases. The present invention provides methods for creating an unsatisfied demand for metal sorption in water-bearing formations in such a way as to effectively remove metal contaminants from groundwater passing through the water-bearing formation.
A water-bearing formation, in many cases referred to as an aquifer, typically is composed of areas through which groundwater flows rapidly, as well as bypassed areas through which water passes more slowly. Water-bearing formations (aquifers) are commonly bounded by relatively impermeable formations, referred to as aquitards. Metal contamination typically enters an aquifer and flows through the most conductive portions, bypassing less conductive areas and aquitards. Metal ions commonly diffuse into the bypassed areas and aquitards, and may sorb there. In any attempt to flush metal contamination from an aquifer, metal is gradually released into the main groundwater flow from bypassed areas and adjoining aquitards. Clean water passing through such a water-bearing formation can become contaminated in this way over an indefinite period. The resulting concentration of contaminants in the water-bearing formation may be small, but nonetheless significant relative to health standards.
There are several sources of heavy metal contamination. These include, but are not limited to, drainage from mining districts, electrical and electronics manufacturing processes, munitions production and weapons laboratories, metal plating processes, battery recycling, coal combustion and fly ash disposal, petroleum refining, chemical production and storage and the nuclear industry. The metal-bearing fluids that carry contamination into a water-bearing formation are commonly acidic.
It has been reported that in the United States, acidic mine drainage affects more than 19,000 kilometers of rivers and streams. Some scientists rate toxic mine drainage as the greatest water-quality problem facing the western United States. It is said that in Colorado alone, the effluent from more than 7,000 abandoned mines contaminates more than 2,500 km of streams. A significant, but unknown, amount of acidic drainage has infiltrated the subsurface, acidying drinking water aquifers there, and contaminating them with metals.
Known methods for remediation of water contaminated with metals include active remediation methods, direct precipitation methods, reactive barriers, and monitored natural attenuation methods.
Active remediation methods for groundwater treatment involve pumping out contaminated water from an aquifer and treatment of the contaminated water to remove the metal contaminants (for example through precipitation processes, sorbent processes, or electrochemical processes). The treated water is, in some cases, returned to the aquifer where it is drawn back toward the water production well. Such pumping facilities require a long-term commitment and the facilities and processes tend to be expensive. A further problem is the disposal of the metal contaminant that had been removed, because the treatment process typically generates large quantities of metal-contaminated waste.
One of the difficulties encountered in the art with active remediation methods of metal attenuation is that the methods do not effectively address the problem of gradual metal release from the water-bearing formation itself. For example, metal contaminants may diffuse from bypassed low conductivity sediment lenses into high conductivity areas.
Water treated by active remediation is, in some cases, pumped back into the aquifer prior to pumping for use. If the problem of metal release from the water-bearing formation itself is not addressed, then metals may diffuse into the previously-treated water, and may be thus rendered again unsuitable for immediate use without further treatment.
Another persistent problem with active remediation is that the difficulty in removing a certain amount of metal contaminant increases significantly with the decrease in metal ion concentration. As a result, it is significantly more costly, in time and money, to treat a large volume of slightly contaminated water than it is to treat a small volume of highly contaminated water.
Direct precipitation methods for groundwater treatment involve precipitation of the metal contaminants within the water source (e.g. aquifer) to keep the metals out of the moving groundwater. Such methods in many cases involve converting the metals to sulfide or other insoluble forms, and both biotic and abiotic approaches have been utilized. Biotic approaches use de-toxifying microorganisms to convert the metals to insoluble granules; such approaches may involve introducing sulfur compounds and stimulating sulfate-reducing bacteria. Abiotic approaches use solution methods to provide ligands and reaction conditions suitable for forming insoluble precipitates. A drawback of this type of approach is that they are able to precipitate only the metal ions present in the aquifer at the time of treatment. An additional difficulty of this method is that the precipitated metals are subject to re-dissolution (for example, by oxidation of sulfide granules), allowing metal ions to once again contaminate water in the aquifer.
Reactive barriers are constructed within a trench dug across an aquifer. The barriers, through which groundwater is allowed to pass, are designed to create a zone of chemical or biological reaction where metal contaminants are immobilized. Drawbacks to reactive barriers include the expense of constructing them, the difficulty in applying them to areas where the contamination is widespread or not limited to shallow depth, the possibility that water flow will bypass them, the possibility that metal concentrations will not be reduced to acceptable levels, and uncertain long-term performance.
Monitored Natural Attenuation methods for groundwater treatment use the naturally-present composition, structure, and microbial content of the aquifer and sediments to immobilize undesirable compounds, such as metal ions. Such methods may be employed following periods of active remediation. Such methods have the potential, where successful, to significantly reduce costs. Such methods for processing groundwater sources run into administrative barriers such as extensive environmental monitoring required for process approval, coupled with a long and expensive application approval process in the United States. In the U.S., there is also political opposition, and the long-term effectiveness of the methods is uncertain.
In natural attenuation methods, the removal of metal contaminants may be dependent upon the sorption capacity of the ground structures that the ground water moves through. When groundwater is infiltrated with acidic fluids, the water-bearing formation becomes acidified, markedly decreasing the effectiveness of natural attenuation methods. The groundwater itself is commonly unable to rapidly affect the pH of the acidified regions in the ground, where metal sorption to the sediments is hindered. Most groundwaters have insufficient alkalinity to rapidly neutralize the sediment surfaces. As a result, the surface acidity remains high during remediation, thus decreasing the effectiveness of the formation in attenuating metal ion concentration. Furthermore, the high surface acidity may permit previously sorbed metals to desorb and reenter the groundwater flow.
Source control refers to processes for control of contaminants from a wastewater source. For example, such treatments may involve precipitation of heavy metals using alkali, and may further include addition of a precipitation agent such as silica, see, e.g. U.S. Pat. Nos. 5,370,827 and 3,579,443. Such treatments do not occur in a groundwater-bearing formation, but are carried out externally, above the land surface.
Flushing techniques are known in the art for flushing undesirable metals from out of land formations that are responsible for contaminating groundwater. U.S. Pat. No. 5,324,433 and concurrent U.S. Pat. No. 5,275,739, describe in situ methods of removing and stabilizing soluble heavy metal contaminants in soil and groundwater. They disclose an ion displacement method of introducing an aqueous remediation solution into a land formation to solubilize, mobilize and remove heavy metal ions from the soil, counteracting the retention of the ions by the charged clays, displacing the heavy metal or radioactive ions with harmless, naturally-occurring ions. The disclosed remediation solution contains at least one remediation ion selected from the group consisting of aluminum, magnesium, calcium, potassium, sodium, hydrogen, chloride, sulfate, carbonate, bicarbonate, hydroxide, or any mixture thereof. After the land formation was sufficiently flushed with remediation solution to decrease effluent undesirable metal ion content, the land formation was treated with a stabilization solution consisting essentially of sodium silicate, potassium silicate, or a mixture thereof to co-precipitate remaining metal contaminants and inhibit their remobilization.
The disclosures of any citations in this description are incorporated herein by reference.
In summary, there are no existing methods for treating a water-bearing formation to improve or regain its ability to sorb metals. There are no methods that effectively address the problem of metals diffusing out of bypassed regions into the main flow of a water-bearing formation. There is a need for an alternative or addition to existing groundwater treatment processes for attenuating the metal contaminant content. There is a great demand for such a process that does not require costly sorbent resins or off-site treatment. There is need for improvement in natural attenuation methods of groundwater treatment.